Hi netgineer,
In this forum post they suggested this fix:
I also experienced this problem when switching my HTTP Gitlab instance over to HTTPS. I had to update my
/etc/gitlab.gitlab-secrets.json
to use HTTPS for the*_endpoint
parameters. I don’t think Mattermost will follow the 302 on the token request if you’re redirecting HTTP to HTTPS. (and rightfully so)
Did you try that?